Hello, my name is Jim and I am a qualified lawyer happy to help you today.
You can refuse payment and tell the debt collector that you are not paying. They will then go away and tell the utility company - who will then start a small claim.
In the meantime you should make a formal complaint and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response.
Once you have the deadlock letter the next step would be to escalate this to Ofgem (their regulator) - they will investigate and liaise with the energy company which would hopefully result in a cancellation with no bill to pay. They can order the energy company to make a financial award for inconvenience if they have acted badly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service which is carried out with care and skill - if you do not then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of the energy company's conduct.
You can complain to the Ombudsman here:
https://www.ombudsman-services.org/sectors/energy or you can send them an email to : [email protected]energy.org
If they sue then the court will send you a claim form and a response pack - you must fill in the response pack to confirm you want to defend the claim and then return it to the court. If you ignored it or delayed then the claimant can request default judgment (which means a county court judgment, or a CCJ). I have attached the response pack so that you know what it looks like when the court sends it to you. The worst-case scenario is the energy company wins, they get £1100 awarded by the court with court fee of £70 and as long as it is paid within 30 days of the court judgment it is not registered with credit agencies - though as I say above, you should not lose.
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.